In the installation of skylight windows in roof structures of the kind comprising plywood plate members secured to the rafters of the structure it is customary practice prior to the installation to cut a hole or an opening through the plate members and the external roof covering secured thereto in order to localize the rafters and avoid damaging when the greater final opening for the window is cut.
After cutting the window opening a curb frame of a form matching the opening is mounted and secured to the roof structure. The curb frame is normally of a rectangular form and composed of four wall members having top edges forming engagement surfaces for part of the window frame structure. Usually the curb is positioned on and secured to rim portions of the roof plate members surrounding the opening.
Following the mounting of the curb a flashing membrane, which is typically made of roof board or metallic flashing members is applied to the external curb surfaces to provide a watertight joint between the curb and the surrounding roof covering.
The skylight window is then arranged in sealing engagement against the top edges of the curb wall members, Typically, the window will be provided with external weather protective covering members which may form a skirt projecting somewhat downwards on the external side of the curb and the flashing membrane applied thereto to provide further protection to the curb and serve to secure the window with respect to the curb by appropriate fastening means, e.g. in the form of nails or screws forced through holes in the skirt into the curb wall members.
Subsequently the curb is usually lined on the internal side of its wall members by suitable panel members like plasterboards forming a shaft extending through the curb as well as the underlying roof structure and insulation. Frequently, this lining operation is made difficult and complicated by the fact that in case of a less accurate cutting of the roof opening parts of the roof plate members may project into the opening at the lower end of the curb, such that a cumbersome individual adaption of the lining members becomes necessary.
Moreover, the need for individual adaption of lining members will frequently reduce the space available for mounting a skylight shade in the form of a venetian blind, a roller blind or a pleated blind, so that the dimensions of standard designs of such accessories will not fit into the available space. Thereby, also accessories which are normally desired for skylight windows may have to be individually adapted, and the window installation will become more complicated and expensive.
U.S. Pat. No. 5,544,455 to DeBlock discloses a skylight frame and curb assembly of the general structure described above, in which the curb provides at least one serrated wall to receive a modular shaft extending from the curb to an opening formed in a ceiling and including a number of panels to each of which a hook with at least one serrated wall is mounted. The hooks of the shaft panels fit into the channel defined by the curb such that the serrated walls contact one another and secure the shaft to the curb by a ratcheting effect.
In this design the need to provide the shaft panel members with special hook members having a serrated wall to engage the serrated wall of the curb complicates the structure without providing any solution to the problems following from a less accurate cutting of the roof opening.
In the curb assembly disclosed in U.S. Pat. No. 5,544,455 a channel for receiving a screening device is formed by flanges projecting from the inner side of the curb frame itself, which together with the need to provide the curb with at least one serrated wall restricts the curb frame to being made from a polymeric material.